15.5 Prepared Statements

15.5.1 PREPARE Statement

15.5.2 EXECUTE Statement

15.5.3 DEALLOCATE PREPARE Statement

MySQL 8.0 provides support for server-side prepared statements. This support takes advantage of the efficient client/server binary protocol. Using prepared statements with placeholders for parameter values has the following benefits:

• Less overhead for parsing the statement each time it is executed. Typically, database applications process large volumes of almost-identical statements, with only changes to literal or variable values in clauses such as WHERE for queries and deletes, SET for updates, and VALUES for inserts.

• Protection against SQL injection attacks. The parameter values can contain unescaped SQL quote and delimiter characters.

The following sections provide an overview of the characteristics of prepared statements:

• Prepared Statements in Application Programs

• Prepared Statements in SQL Scripts

• PREPARE, EXECUTE, and DEALLOCATE PREPARE Statements

• SQL Syntax Permitted in Prepared Statements

Prepared Statements in Application Programs

You can use server-side prepared statements through client programming interfaces, including the MySQL C API client library for C programs, MySQL Connector/J for Java programs, and MySQL Connector/NET for programs using .NET technologies. For example, the C API provides a set of function calls that make up its prepared statement API. See C API Prepared Statement Interface. Other language interfaces can provide support for prepared statements that use the binary protocol by linking in the C client library, one example being the mysqli extension, available in PHP 5.0 and higher.

Prepared Statements in SQL Scripts

An alternative SQL interface to prepared statements is available. This interface is not as efficient as using the binary protocol through a prepared statement API, but requires no programming because it is available directly at the SQL level:

• You can use it when no programming interface is available to you.

• You can use it from any program that can send SQL statements to the server to be executed, such as the mysql client program.

• You can use it even if the client is using an old version of the client library.

SQL syntax for prepared statements is intended to be used for situations such as these:

• To test how prepared statements work in your application before coding it.

• To use prepared statements when you do not have access to a programming API that supports them.

• To interactively troubleshoot application issues with prepared statements.

• To create a test case that reproduces a problem with prepared statements, so that you can file a bug report.

PREPARE, EXECUTE, and DEALLOCATE PREPARE Statements

SQL syntax for prepared statements is based on three SQL statements:

• PREPARE prepares a statement for execution (see Section 15.5.1, “PREPARE Statement”).

• EXECUTE executes a prepared statement (see Section 15.5.2, “EXECUTE Statement”).

• DEALLOCATE PREPARE releases a prepared statement (see Section 15.5.3, “DEALLOCATE PREPARE Statement”).

The following examples show two equivalent ways of preparing a statement that computes the hypotenuse of a triangle given the lengths of the two sides.

The first example shows how to create a prepared statement by using a string literal to supply the text of the statement:

mysql> PREPARE stmt1 FROM 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
mysql> SET @a = 3;
mysql> SET @b = 4;
mysql> EXECUTE stmt1 USING @a, @b;
+------------+
| hypotenuse |
+------------+
|          5 |
+------------+
mysql> DEALLOCATE PREPARE stmt1;

The second example is similar, but supplies the text of the statement as a user variable:

mysql> SET @s = 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
mysql> PREPARE stmt2 FROM @s;
mysql> SET @a = 6;
mysql> SET @b = 8;
mysql> EXECUTE stmt2 USING @a, @b;
+------------+
| hypotenuse |
+------------+
|         10 |
+------------+
mysql> DEALLOCATE PREPARE stmt2;

Here is an additional example that demonstrates how to choose the table on which to perform a query at runtime, by storing the name of the table as a user variable:

mysql> USE test;
mysql> CREATE TABLE t1 (a INT NOT NULL);
mysql> INSERT INTO t1 VALUES (4), (8), (11), (32), (80);

mysql> SET @table = 't1';
mysql> SET @s = CONCAT('SELECT * FROM ', @table);

mysql> PREPARE stmt3 FROM @s;
mysql> EXECUTE stmt3;
+----+
| a  |
+----+
|  4 |
|  8 |
| 11 |
| 32 |
| 80 |
+----+

mysql> DEALLOCATE PREPARE stmt3;

A prepared statement is specific to the session in which it was created. If you terminate a session without deallocating a previously prepared statement, the server deallocates it automatically.

A prepared statement is also global to the session. If you create a prepared statement within a stored routine, it is not deallocated when the stored routine ends.

To guard against too many prepared statements being created simultaneously, set the max_prepared_stmt_count system variable. To prevent the use of prepared statements, set the value to 0.

SQL Syntax Permitted in Prepared Statements

The following SQL statements can be used as prepared statements:

ALTER TABLE
ALTER USER {DEFAULT ROLE}
ANALYZE TABLE
CACHE INDEX
CALL
CHANGE {MASTER | REPLICATION FILTER}
CHECKSUM
COMMIT
{CREATE | DROP} INDEX
{CREATE | RENAME | DROP} DATABASE
{CREATE | DROP} TABLE
{CREATE | RENAME | DROP} USER
{CREATE | DROP} VIEW
DELETE
DO
FLUSH
GRANT {ROLE}
INSERT
INSERT ... SELECT
INSTALL PLUGIN
KILL
LOAD INDEX INTO CACHE
OPTIMIZE TABLE
RENAME TABLE
REPAIR TABLE
REPLACE
RESET {MASTER | REPLICA}
REVOKE {ALL | ROLE}
SELECT
SET {PASSWORD | RESOURCE GROUP | ROLE | VARIABLE}
SHOW {BINLOG EVENTS | BINARY LOGS | CHARACTER SETS | COLLATIONS | DATABASES | ENGINE |
      ERRORS | EVENTS | FIELDS | FUNCTION CODE | FUNCTION STATUS | GRANTS | KEYS | OPEN TABLES |
      PLUGINS | PRIVILEGES | PROCEDURE CODE | PROCEDURE STATUS | PROCESSLIST | PROFILE | PROFILES | 
      RELAYLOG EVENTS | REPLICAS | REPLICA STATUS | STATUS | PROCEDURE STATUS | TABLE STATUS | TABLES | 
      TRIGGERS | VARIABLES | WARNINGS}
SHOW CREATE { DATABASE | EVENT | FUNCTION | PROCEDURE | TABLE | TRIGGER | USER | VIEW}
REPLICA {START | STOP}
TRUNCATE
UNINSTALL PLUGIN
UPDATE

Other statements are not supported.

For compliance with the SQL standard, which states that diagnostics statements are not preparable, MySQL does not support the following as prepared statements:

• SHOW WARNINGS, SHOW COUNT(*) WARNINGS

• SHOW ERRORS, SHOW COUNT(*) ERRORS

• Statements containing any reference to the warning_count or error_count system variable.

Generally, statements not permitted in SQL prepared statements are also not permitted in stored programs. Exceptions are noted in Section 27.8, “Restrictions on Stored Programs”.

Metadata changes to tables or views referred to by prepared statements are detected and cause automatic repreparation of the statement when it is next executed. For more information, see Section 10.10.3, “Caching of Prepared Statements and Stored Programs”.

Placeholders can be used for the arguments of the LIMIT clause when using prepared statements. See Section 15.2.13, “SELECT Statement”.

In prepared CALL statements used with PREPARE and EXECUTE, placeholder support for OUT and INOUT parameters is available beginning with MySQL 8.0. See Section 15.2.1, “CALL Statement”, for an example and a workaround for earlier versions. Placeholders can be used for IN parameters regardless of version.

SQL syntax for prepared statements cannot be used in nested fashion. That is, a statement passed to PREPARE cannot itself be a PREPARE, EXECUTE, or DEALLOCATE PREPARE statement.

SQL syntax for prepared statements is distinct from using prepared statement API calls. For example, you cannot use the mysql_stmt_prepare() C API function to prepare a PREPARE, EXECUTE, or DEALLOCATE PREPARE statement.

SQL syntax for prepared statements can be used within stored procedures, but not in stored functions or triggers. However, a cursor cannot be used for a dynamic statement that is prepared and executed with PREPARE and EXECUTE. The statement for a cursor is checked at cursor creation time, so the statement cannot be dynamic.

SQL syntax for prepared statements does not support multi-statements (that is, multiple statements within a single string separated by ; characters).

To write C programs that use the CALL SQL statement to execute stored procedures that contain prepared statements, the CLIENT_MULTI_RESULTS flag must be enabled. This is because each CALL returns a result to indicate the call status, in addition to any result sets that might be returned by statements executed within the procedure.

CLIENT_MULTI_RESULTS can be enabled when you call mysql_real_connect(), either explicitly by passing the CLIENT_MULTI_RESULTS flag itself, or implicitly by passing CLIENT_MULTI_STATEMENTS (which also enables CLIENT_MULTI_RESULTS). For additional information, see Section 15.2.1, “CALL Statement”.